<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style type="text/css">
    * {
        padding: 0;
        margin: 0;
    }

    li {
        list-style: none;
    }

    body {
        background: #f6f9fc;
        font-family: arial;
    }

    .calendar {
        width: 210px;
        margin: 0 auto;
        padding: 10px 10px 20px 20px;
        background: #eae9e9;
    }

    .calendar ul {
        width: 210px;
        overflow: hidden;
        padding-bottom: 10px;
    }

    .calendar li {
        float: left;
        width: 58px;
        height: 54px;
        margin: 10px 10px 0 0;
        border: 1px solid #fff;
        background: #424242;
        color: #fff;
        text-align: center;
        cursor: pointer;
    }

    .calendar li h2 {
        font-size: 20px;
        padding-top: 5px;
    }

    .calendar li p {
        font-size: 14px;
    }

    .calendar .active {
        border: 1px solid #424242;
        background: #fff;
        color: #e84a7e;
    }

    .calendar .active p {
        font-weight: bold;
    }

    .calendar .text {
        width: 178px;
        padding: 0 10px 10px;
        border: 1px solid #fff;
        padding-top: 10px;
        background: #f1f1f1;
        color: #555;
    }

    .calendar .text h2 {
        font-size: 14px;
        margin-bottom: 10px;
    }

    .calendar .text p {
        font-size: 12px;
        line-height: 18px;
    }
</style>

</head>

<script>
    var arr=['快过年了，大家可以商量着去哪玩吧～',
		'大家好好学习吧222222~~~',
		'大家好好学习吧222222333~~~',
		'大家好好学习吧222444222~~~',
		'大家好好学习555吧222222~~~',
		'大家好好学习吧666222222~~~',
		'大家好好学习吧227772222~~~',
		'大家好好学习吧28888822222~~~',
		'大家好好学习吧99999222222~~~',
		'大家好好学习10000000吧222222~~~',
		'大家好好学习吧111111222222~~~',
		'大家好好学习吧22222200000000000~~~']
       
    window.onload = function () {
       
        var list = document.getElementsByTagName("li");
        var txt = document.getElementById("txt");
        var h2  = txt.getElementsByTagName("h2")[0];
        var desc = txt.getElementsByTagName("p")[0];

        //清除所有，设置按下元素的样式
        for (var i = 0; i < list.length; i++) {
            list[i].onclick = function () {  
                for (var j = 0; j < list.length; j++) {
                list[j].className = "";
                }
                this.className = "active";
                h2.innerText = this.i +"月活动";
                    
            }
        }
    }

</script>

<body>
    <div id="tab" class="calendar">
        <ul id="box">
            <li class="active">
                <h2>1</h2>
                <p>JAN</p>
            </li>
            <li >
                <h2>2</h2>
                <p>FER</p>
            </li>
            <li>
                <h2>3</h2>
                <p>MAR</p>
            </li>
            <li>
                <h2>4</h2>
                <p>APR</p>
            </li>
            <li>
                <h2>5</h2>
                <p>MAY</p>
            </li>
            <li>
                <h2>6</h2>
                <p>JUN</p>
            </li>
            <li>
                <h2>7</h2>
                <p>JUL</p>
            </li>
            <li>
                <h2>8</h2>
                <p>AUG</p>
            </li>
            <li>
                <h2>9</h2>
                <p>SEP</p>
            </li>
            <li>
                <h2>10</h2>
                <p>OCT</p>
            </li>
            <li>
                <h2>11</h2>
                <p>NOV</p>
            </li>
            <li>
                <h2>12</h2>
                <p>DEC</p>
            </li>
        </ul>

        <div class="text" id="txt">
            <h2>1月活动</h2>
            <p>快过年了，大家可以商量着去哪玩吧～</p>
        </div>

    </div>
</body>

</html>